Changes in blood flow velocity through the tumors can induce damage of tumor microcirculation and thus may contribute to the final destruction of tumor masses after photodynamic therapy (PDT). The aim of this study was to evaluate the blood flow changes in a SCCVII mouse carcinoma during Photofrin-based photodynamic therapy by analyzing several quantitative spectral Doppler parameters [maximum systolic flow velocity (Vmax), end diastolic velocity (Vmin), resistance index (RI) and pulsatile index (PI)] by using the color Doppler ultrasonography. Blood flow velocities were recorded immediately prior to tumor illumination (0 h) and then 2 and 24 h after the illumination. Statistically significant increase in diastolic blood velocity (Vmin) with a corresponding decline in RI and PI was recorded in tumors of the Photofrin-injected mice prior to tumor illumination. However, 2 h after the illumination a pronounced decrease in both Vmin and Vmax was obtained. There were no changes of these parameters in controls at different times during determination. The observed changes of spectral Doppler parameters in tumors from the PDT group point to the transition of tumor blood vessels from the relaxation state recorded before tumor illumination into a state of increased contraction after the activation of Photofrin by light. Pronounced changes in tumor blood vessel tone might be an additional stress for such vessels leading to their ultimate destruction.